The invention concerns a fuel cell system with a fuel cell displaying an anode and a cathode, with a system for supplying a fuel-water mixture to the anode and a system for supplying an oxidant to the cathode as well as with a system for carrying off the liquid-vapor mixture occurring at the anode of the fuel cell, said removal system consisting of two parallel-running paths each with a vapor separator, with a mainstream cooler being connected in front of the vapor separator in the first path (low temperature path) so that the liquid-vapor mixture is cooled when fed to the vapor separator, and in which the second path (high temperature path) the liquid-vapor mixture is supplied essentially uncooled to the vapor separator and with a crossover path between the two paths through which the gas separated in the vapor separator in the high temperature path is fed cooled to the vapor separator in the low temperature path.
Such a system is described in DE 197 01 560 A1: The central feature of the system is a fuel cell which is powered directly by methanol, i.e. the anode of the cell is supplied a mixture of methanol and water, with the methanol reacting chemically at the anode. At this time among others, carbon dioxide is formed. As the oxidizing agent, an oxygen-containing gas, preferably ambient air, is supplied to the cathode. The term xe2x80x9cfuel cellxe2x80x9d in this case refers not just to a single cell but rather to a system of several cells connected to each other, which is referred to by the technical term xe2x80x9cstackxe2x80x9d.
The systems of this type encounter the following problem: the fuel cell is operated superstoichiometrically, i.e. only a small part of the methanol supplied to the anode reacts with water to form carbon dioxide. Therefore, a liquid-vapor mixture is present at the fuel cell outlet, with methanol, carbon dioxide and water being present in both the liquids as well as in the vapor phase. The unconsumed methanol and water contents are returned to the inlet of the anode in a circulation after the prior separation of the carbon dioxide. This takes place in so-called vapor separators, in which case, however, it must be assured that too much water and methanol are not discharged with the Co2 in the form of gas.
It has been proposed that the liquid-vapor mixture be supplied in cooled form to the vapor separator so that the highly volatile methanol remains essentially in the liquid phase. However, it has been found that the problem of excessive methanol discharge cannot be satisfactory resolved in such a system
Therefore, in DE 197 01 560 A1 a system is proposed in which the return of the methanol takes place via two parallel-running paths in each of which a vapor separator is present. In a high temperature path a part of the liquid-vapor mixture which emerges from the fuel cell at a temperature of ca. 80-130xc2x0 C. is fed to a first vapor separator (high temperature vapor separator).
The other part of the liquid-vapor mixture is fed, cooled, to a second vapor separator (low temperature vapor separator) at which time before cooling the still hot liquid-vapor mixture is mixed in this path with the vapor emerging from the high temperature separator. The mixing is accomplished in a metered way so that, according to the patent disclosure, the mass streams and with them, the temperature level can be selectively influenced so that variable control and regulating procedures are realizable (column 3, lines 46 ff.). The disadvantage here is that no optimal values can be achieved with respect to the methanol output so that water must be supplied in addition from the cathode circulation tot he low temperature vapor separator in order to be able to wash out the methanol still present in the vapor of the low temperature vapor separator.
To avoid this problem, the present invention proposes that a branch-stream cooler be provided in the crossover path so that the vapor emerging from the high temperature vapor separator is returned to the low temperature path cooled, and the condensate accumulating in the branch-stream cooler is returned to the high temperature vapor separator.
In particular, the crossover path should open into the low temperature vapor separator or into the liquid accumulation consisting essentially of methanol and water at the bottom of the vapor separator, i.e. below the liquid level formed by the liquid accumulation.
That part of the liquid-vapor mixture which goes into the high temperature path passes into the high temperature vapor separator. Carbon dioxide, gaseous water and gaseous methanol corresponding to the phase equilibrium are separated from the liquid at the bottom of the expansion chamber into the expansion chamber of the vapor separator which is located above the liquid level. This gas stream is cooled by the second branch-stream cooler. The methanol-containing liquid condenses out at this time is returned to the high temperature vapor separator. The emerging gas stream is brought into contact in the low temperature vapor separator with the cooled liquid accumulation at the bottom of the low temperature vapor separator. In this way a material exchange takes place in which the methanol and water vapor from the gas stream of the crossover path pass over into the cold liquid from the low temperature path. In this way the gaseous methanol can be brought back into the liquid phase and returned to the circulation. Depending on what the volume ratio between the two paths is, the effective methanol output can be kept very small.
In order to reduce it even further, it is recommended that another vapor separator or a scrubber be provided at the gas outlet of the low temperature vapor separator which is operated with water from the cathode cycle.